It is well known to make sintered products by compacting a plurality of iron particles in a die to form an unsintered, so-called "green", compact, and then heating the green compact in a protective atmosphere at a suitable temperature for a time sufficient to effect solid state bonding (i.e., sintering) of the particles to each other. Compaction may be uniaxial or isostatic. In uniaxial compaction, the particles are placed in a die and pressed in one direction by a punch. In isostatic compaction, the particles are placed in a flexible mold/container (e.g., rubber bag, sheet metal can, etc.), submerged in a pressurized fluid (i.e., gas or liquid) pressing medium, and pressed in all directions either at ambient or at elevated temperatures. One such isostatic compaction process using a liquid pressing medium is known as the HIP, which stands for "Hot Isostatic Pressing". Another such isostatic process using a gas pressing medium is known as the PIF process, which stands for "Pneumatic Isostatic Forging".
Known variations of the aforesaid sintering process include such additional steps as: (a) mixing lubricants with the particles, and heating the particles (e.g., 1400.degree. F.-1600.degree. F.) to drive off the lubricants (i.e., "delubing") between the compaction and sintering steps; (b) repressing and resintering the sintered compact following initial sintering; and (c) isostatically compacting the sintered compact to further densify it. The PIF process has been used to so densify sintered compacts. To densify a sintered compact using the PIF process, the as-sintered compact has heretofore been: (a) cooled down to ambient temperature; (b) encased in a shell which seals its outer surface against penetration of the gaseous pressing medium into the bowels of the sintered compact; (c) heated back up to the sintering temperature; and then (d) surrounded by, and subjected to, pressing gas pressures sufficiently high (i.e., ca. 10,000 psi to ca. 60,000 psi) as to densify the sintered compact. The sealing shell may take several forms including, (1) packaging the compact in an evacuated thin flexible sheet metal can/mold, (2) applying a sealant (e.g., molten glass or electroless nickel) to the surface of the compact to seal the surface pores, and (3) shot peening the surface of the sintered compact to mechanically close the pores at the surface.
Isostatic compacting processes are very costly due to long cycle times including cooling and reheating steps, high labor and energy content, and the need to package, or seal the surface of, the compact. The technique of the present invention is a cost effective improvement to the PIF process which utilizes an oxide sealant grown in situ on the surface of the compact at an elevated temperature in lieu of packaging, or otherwise sealing the surface of the compact. The technique contemplates a continuous process wherein the compact moves on a belt through an elongated furnace having different regions/chambers for sequentially effecting the different operations while eliminating unnecessary cooling and handling of the compact midway in the process, and eliminating the need for costly sealing materials and the labor to apply them.